As their name indicates, these nymphs are daughters of Nereus, the Old Man of the Sea. These minor sea-goddesses, who represent the sea at calm, are generally treated as a group, lacking individualizing traits. Two important exceptions, however, are Amphitrite, Poseidon's wife, and especially Thetis, Achilles' mother, who is a major deity.

Nereids were imagined as beautiful young women, and share many traits with other varieties of nymphs, e.g., love for playing and dancing (often with marine creatures, such as dolphins), and relations with men (not always happy, as Thetis' marriage to Peleus shows), whom they help when sailing and accompany to safety. From this derives a second function: Nereids escort humans in important life transitions, especially marriage and death (Barringer 1991, 661-662). They are thus frequently present in funerary contexts, where they perform as a chorus of mourners, an aspect highlighted at Iliad 18.35-69, where they accompany their sister Thetis in her lamentation for Achilles, after Patroklos' death (cf. Il .24.78-84). Thetis herself is cast as a mater dolorosa from the beginning to the end of the Iliad.

Alerted by Thetis' mourning cries, the Nereids gather in a cave where they join her in her threnody. Though it is Patroklos who is dead, their grief is in fact for Achilles, as Thetis' words (18.51-64) make clear. Language (κώκυσεν, 18.37; ἐξῆρχε γόοιο 18.51) and gestures, such as beating their breasts (50-51), are specific to the mourning rituals (see Lament). Homer may have modeled this scene on the Cyclic Aethiopis, where Thetis and the Nereids would in fact have been bewailing Achilles' death (Kakridis 1949, 65-95), in counterpoint to the male chorus of Myrmidons (cf. Od. 24.40-64). In their singing and connection with death they resemble the Sirens (Bader 1994).

Their number varies in different authors. The Homeric catalogue (Il. 18.35-49) names thirty-three (but adds “and others”). Zenodotus and Aristarchus athetized the passage (see Athetesis), which overlaps largely with Hesiod's longer list (Th .240-264; cf. schol. Il .18.39-49), although a number of such lists were very likely in circulation, and Hesiod's and Homer's catalogues may have the same source. Other catalogues are found in Apollodorus' Library 1.11 (forty-five) and Hyginus' Fabulae Praef .8 (fifty).

Nereids were a popular motif in art, from the Archaic to the Roman period. In most cases they are represented as bringing arms to Achilles. It is debated whether these arms are those that Achilles brought to Troy, or the second set of armor which he obtained in Troy, after Patroklos' death. In the Iliad Thetis alone delivers the new armor for her son (19.4), but the Nereids may have figured in the lost Achilleia of Aeschylus. There are many representations of the Nereids as companions of Europa, as she is carried away by Zeus in the form of a bull; this may reflect simultaneously a transition of marriage and to death, which are often paired in Greek culture (Barringer 1991; Shapiro 1991).